Structure and evolution of the C. elegans embryonic endomesoderm network

Abstract

The specification of the Caenorhabditis elegans endomesoderm has been the subject of study for more than 15 years. Specification of the 4-cell stage endomesoderm precursor, EMS, occurs as a result of the activation of a transcription factor cascade that starts with SKN-1, coupled with input from the Wnt/beta-catenin asymmetry pathway through the nuclear effector POP-1. As development proceeds, transiently-expressed cell fate factors are succeeded by stable, tissue/organ-specific regulators. The pathway is complex and uses motifs found in all transcriptional networks. Here, the regulators that function in the C. elegans endomesoderm network are described. An examination of the motifs in the network suggests how they may have evolved from simpler gene interactions. Flexibility in the network is evident from the multitude of parallel functions that have been identified and from apparent changes in parts of the corresponding network in Caenorhabditis briggsae. Overall, the complexities of C. elegans endomesoderm specification build a picture of a network that is robust, complex, and still evolving.

FIG. 1

The C. elegans endomesoderm gene regulatory network. Ovals represent transcription factors, while rectangles indicate other types of proteins. Question marks denote hypothesized co-regulators or functions. Arrows denote, in most cases, direct regulatory interactions. Thinner arrows denote contributions that are hypothesized to be weaker based on genetic evidence. Overlapping transcription factor symbols denote common function and are not meant to imply physical interaction. Diagrams of the C. elegans embryo (4-cell and 8-cell stages), and anatomy of the digestive tract in a larva are shown, after ref. [3], with anterior to the left, and dorsal upwards. Sections of the digestive tract are labeled with the name of the blastomere whose descendants contribute to that region. The association of ‘anterior pharynx’ with ABa, and ‘posterior pharynx’ with MS is in reality not a precise distinction [9] and is shown here as such for simplicity. GLP-1/Notch-dependent cell-cell interactions are shown as ‘Notch’ with an arrow [9].

FIG. 2

Appearance of wild-type and zygotic cell fate specification mutants and associated expression of a pha-4::GFP reporter (pseudocolored yellow; a gift from Jeb Gaudet, University of Calgary). (A) Wild-type L1, showing pharynx and rectum (red) and intestine (blue) and associated blastomeres as in Fig. 1. (B) Arrested end-1(ok558) end-3(ok1448) double mutant embryo, showing absence of gut and slightly reduced body length as compared with wild-type larvae. The vast majority of end-1,3(−) embryos elongate within the eggshell, but only some of these hatch. A more detailed characterization of these mutants will be presented elsewhere. (C) Arrested med-1(ok804); med-2(cx9744); end-1(ok558) end-3(ok1448) quadruple-null mutant. The most severely affected med-1,2(−) embryos arrest lacking gut and at a similar stage as the embryo shown here [25]. (D–F) Expression of pha-4::GFP in pharynx, rectum and intestine cells, corresponding to the DIC images in A–C. The hypodermis is indicated with a dotted blue line. Anterior is to the left in panels A, B, E and F, and up in panels C and F. All micrographs are shown at the same scale. In A–C, Adobe Photoshop was used to colorize regions. A C. elegans embryo is approximately 50μm along its long axis.

FIG. 3

Speculative models for stepwise evolution of two extant motifs in the C. elegans endomesoderm network, starting with single activator, single target interactions. (A) Stepwise formation of the SKN-1 → MED-1,2 → TBX-35 regulatory chain. (B) Formation of a feed-forward loop involving SKN-1 and two ENDs. Note that autoregulation of the first end gene is inferred but this is not known for end-1. Intercalation of the MEDs could occur at an early or later step by acquisition of MED sites in the end genes (not shown).